The present invention relates to an equipment for optically measuring a height of a step, and particularly to an equipment for measuring a height of a step using an optical fiber suitable for noncontact measurement of a minute amount of the height of the step, and it is applied, for instance, to the measurement of the height of the step formed on a semiconductor wafer when semiconductor devices are manufactured.
One of prior-art methods for noncontact measurement of a minute height of a step is discussed in the Applied Optics, Vol. 20 (1981), pp. 610 to 618. The method discussed therein utilizes beat signals of laser beams oscillated at different frequencies. By utilizing the Zeeman effect, a helium-neon (He-Ne) laser can be made to oscillate simultaneously laser beams slightly different in wavelength from each other. These laser beams different in wavelength have planes of polarization perpendicular to each other.
The laser beams are focused on the surface of an object by an optical system whose sectional structure is illustrated in FIG. 4. A Wollaston prism 41 employed as a component in the optical system of FIG. 4 refracts incident beams having planes of polarization perpendicular to each other, in the different directions respectively. Because of this refraction, the laser beams are focused at two positions on the surface of an object 43 by an objective lens 42, and the focused positions are separated according to the planes of polarization of the laser beams. The light reflected by the surface of the object is transmitted again through the focusing objective lens 42, and optical paths having been separated are made to coincide again by the operation of the Wollaston prism 41. The incident light in the Wollaston prism 41 contains laser beams different in frequency, and therefore, when a light having some component plane of polarization is taken out by a polarizer, the laser beam having the component plane of polarization generates a beat of a frequency which is equal to the difference in frequency between the laser beams.
The beams having the component planes of polarization perpendicular to each other advance along separate optical paths, which coincide again by the Wollaston prism 41 which previously separated the paths. When these two optical paths are equal in length, the phase of the beat generated from the reflected light is equivalent to that of the beat of the laser beams incident in the Wollaston prism 41. In the case when the optical paths are different in length in the course from separation to coincidence thereof, however, a change occurs in the phase of the beat. This change in the phase of the beat is caused by the difference in length between the optical paths, and this difference, in its turn, results from the unevenness of the surface of an object. Therefore, the change in the phase is measured, and the unevenness of the surface of the object is calculated from values obtained from the measurement.
One example of a method of measuring a minute change in the height by using an optical fiber is shown in the Applied Physics Letter, Vol. 41 (1982), pp. 231 to 233. According to this method, laser beams separated in two by a directional coupler are projected onto the surface of a sample with a time difference between them, and beams reflected therefrom are coupled again by the directional coupler so that they interfere with each other. The change in the height of the surface of the sample appearing within the time difference in application of the beams onto the sample is measured from a change in the intensity of the laser beam caused by the interference.
Out of the above-described prior arts, the former requires the fixation on an optical base of the components constituting an optical system so as to prevent the mutual spacing of the components from being varied by vibrations. This results in enlarged dimensions of equipment, an increased weight thereof, causing a problem in employment thereof. Moreover, this is a method utilizing a Zeeman effect, and therefore it requires the so-called Zeeman laser oscillated in two frequencies, which produces a problem that an expensive equipment is needed.
The latter of said prior arts can not be applied to the measurement of the difference in height of the surface of an object, since it can measure only an amount changing with time, although it has succeeded in making equipment small in size and light in weight by the use of the optical fiber.